Android: The calculator that comes with stock Android does a decent job of simple calculations. But if you're looking for something more powerful, this modified version brings all manner of advanced options.
The app is a modified version of the stock calculator by Xlythe. It comes included in CyanogenMod standard (yet another reason to try a custom ROM), but anyone can install this version.
Even if you're not after the fancy graphing features, this one gets you one step closer to stock Android without rooting your phone.